The Crab nebula is a case of a remnant without its shell; it is still an open issue why there is no detected shell surrounding the Crab nebula.
Recent observations by the Hitomi satellite showed
that the thermal X-ray emission around the Crab nebula is significantly fainter than other young SNRs
[1]}.
The total ejecta mass is estimated to be less than \(\sim 1M_\odot \) ,
implying that the progenitor explosion was relatively low energy.
One possibility is that
the Crab nebula is a remnant of an electron capture supernova [2]}.
More detailed observations for the Crab and more samples are needed
to understand the origin of plerion-type SNRs.
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